Late rains rally Marin's endangered coho

Marin's endangered coho went from bust to semi-boom this winter despite a dearth of rain when the species needed it most.

December and January rains help facilitate the return of the coho from the sea to their spawning grounds. The coho have a three-year life cycle that sees them born, travel to open sea, then almost miraculously return to their creeks of birth where they spawn and die.

The rains help swell the main stem of Lagunitas Creek and its tributaries, allowing the coho areas in which to spawn. Studies and observation show that it's largely Mother Nature's rain that triggers the return of the coho.

But the rains did not come.

December saw only 1.16 inches of rain and January was even worse: .01 of an inch, an all-time low dating back to 1879. The previous January low was 0.31 of an inch set in 1976.

The initial lack of rain hurt the return: Aside from a population crash of the species in 2008-09, the number of coho and their egg nests seen in December were the fewest in 17 years, since their numbers were first recorded by the Marin Municipal Water District.

But the rains did start to come in early February and while it was late in the coho season, it helped do the trick.

"Coho season is wrapping up, and thankfully it's ending with more of a bang than a whimper," said Eric Ettlinger, aquatic ecologist with the water district.

The preliminary watershed totals show 433 fish and 203 redds, or egg nests. Those numbers are roughly double the size of the coho run three years ago. That is compared to 156 fish and 59 redds counted at the end of January.

"The rain started falling ... and coho spawning took off," Ettlinger said.

The species was said to be in an "extinction vortex" after numbers dipped to all-time lows four years ago, but numbers have been on the rise since then and it was believed — with some good rain — this could be a strong year for the fish.

While Ettlinger was pleased to see the last-minute run of coho he has noted the numbers might have been even bigger if there had been more rains in December and January.

"Earlier rains would have helped some waterfalls to flow to allow fish to get up and over them to get into the San Geronimo Valley," he said. "It would have opened the valley up, but that didn't happen soon enough."